1. Field of the Invention
In one of its aspects, the present invention relates to a fluid mixing device. In another of its aspects, the present invention relates to a method for mixing a fluid.
2. Description of the Prior Art
Mixing devices are known in the art and have been used to promote fluid turbulence—for example, to improve contact between elements in the flow path. Industrial applications of mixing are widely varied, and include heat exchange, reactor engineering and non-reactive blending.
One specific area of reactor engineering where mixing has been used is in the design of fluid treatment devices, particularly fluid radiation treatment devices. A specific such fluid radiation treatment device includes ultraviolet (UV) disinfection devices for water and wastewater treatment. The performance of UV disinfection devices depends, at least in part, on providing a prescribed dose of UV radiation to all fluid elements passing through (or otherwise being treated by) the device.
The UV dose received by a fluid element is defined as the product of UV intensity and exposure time. The accumulated UV dose received by a fluid element exiting the device is the sum of the individual doses received at each position. Since the UV intensity is attenuated with the distance from the UV source, it is desirable to mix fluid elements from regions far from the UV source to regions of higher intensity nearer to the source, thereby ensuring they receive an adequate dose of UV radiation. This type of mixing is particularly desirable when the transmittance of UV radiation through the fluid being treated is low, causing an increase in the attenuation of UV intensity with distance from the source—this is commonly encountered in UV disinfection devices for the treatment of wastewater.
U.S. Pat. No. 5,846,437 [Whitby et al. (Whitby)], assigned to the assignee of the present application, teaches turbulent mixing in a UV system. More specifically, Whitby teaches the use of one or more ring-shaped devices (e.g., washers) at predetermined locations on the exterior surface of each lamp unit in the system and/or ring-shaped devices upstream of each lamp unit to increase turbulent mixing of fluid passing by the lamp units. While the use of such ring-shaped devices as taught in Whitby is useful in increasing turbulence between the lamp units, the turbulent flow of fluid tends to be of a random or non-ordered (e.g., isotropic) nature.
In many systems, such as those where the mixing zone is longitudinal with respect to the direction of fluid flow therethrough, it is desirable to have plug flow in the axial direction and effective mixing in the radial direction. A specific or ordered pattern of fluid flow in the mixing zone is desirable (e.g., a “particle” of fluid oscillating toward and away from the lamp as it passes longitudinally with respect thereto), which is in contrast to general mixing in all directions (i.e., in contrast to random mixing or turbulence taught by Whitby). A longitudinal vortex is an example of this type of flow pattern. Vortices can be formed actively through energy input to the fluid, such as by employing a motorized fluid impeller.
Another means of achieving vortex generation is through the use of a passive element which is designed to cause the formation of the desired flow pattern (vortex generator).
U.S. Pat. Nos. 5,696,380, 5,866,910 and 5,994,705 [all in the name Cooke et al. (Cooke)] teach a flow-through photochemical reactor. The subject reactor taught by Cooke comprises an elongate annular channel in which is disposed an elongate radiation source. The channel includes static, fluid-dynamic elements for passively inducing substantial turbulent flow within the fluid as it passes through the channel. According to Cooke, each such static, fluid-dynamic element advantageously creates a pair of “tip vortices” in the fluid flow past each element. The “tip vortices” purportedly are counter-rotating about an axis parallel to the elongate annular chamber.
U.S. Pat. No. 6,015,229 [Cormack et al. (Cormack)], assigned to the assignee of the present application, teaches a fluid mixing device. The fluid mixing device comprises a series of “delta wing” mixing elements which cause the formation of vortices thereby improving fluid mixing. A specific embodiment of such a device illustrated in Cormack is the use of “delta wing” mixing elements to cause such vortex mixing between UV radiation sources in an array of such sources. This creates the potential for increasing distance between adjacent UV radiation sources in the array which, in turn, allows for a reduction in hydraulic head loss of the fluid flow through a UV disinfection system comprising the fluid mixing device.
Despite the advances in the art made by Cooke and Cormack, there is still room for improvement. For example, there is an ongoing need for fluid mixing devices which, when used in fluid treatment devices such as UV disinfection systems, are capable of improving the UV radiation dose equivalent (this term will be described in more detail hereinbelow) delivered by the disinfection system to the fluid being treated.
Further, the vortex generating devices taught by Cooke and Cormack use the kinetic energy of the flowing fluid to cause mixing. This necessarily results in a loss of fluid pressure, or fluid head in an open channel system. This is very undesirable in UV disinfection systems, particularly those used to treat municipal wastewater, since the UV disinfection system typically is the last station of the multi-station treatment plant. As such, wastewater entering the treatment plant typically suffers hydraulic head loss as it passes from station to station with the result that, when the wastewater reaches the UV disinfection system, there is not much room for further significant loss of hydraulic head. Accordingly, it would be highly desirable to have a fluid mixing device which, in addition to improving dose equivalent as described above when used in a UV disinfection system, resulted in reducing the hydraulic head loss of fluid being treated by the UV disinfection system.